1. Field of the Invention
The present invention relates generally to lithographic production of microelectronic circuits or other features and, more particularly, to a method and apparatus for storing and transporting masks used in lithographic production.
2. Description of Related Art
In manufacturing integrated circuits and other features using lithographic techniques, it is a goal to build a defect-free reticle or photomask with the desired patterns which then are projected and exposed onto the resist layers on a wafer during lithographic processing to create the circuits. Particle contaminants on reticles or photomasks can cause problems during lithographic imaging. Particles on the front (patterned) side of the mask are imaged on the water and will cause yield loss. This problem has been mitigated in the past by placing a protective pellicle over the front side of the mask, so that any particles are deposited on the pellicle instead of the mask itself. The pellicle is transparent to the exposure wavelength and keeps particles out of focus so that they do not image on wafers. Particles on the back side of the mask are unlikely to image, because they are already 0.25 in. (6.35 mm) away from the focal plane. However, even these back-side particles are undesirable since large ones may alter the exposure light, and because the particles could move to other undesired locations. An additional concern is introduced by small particles which can form a haze that can alter mask transmission. For applications requiring back-side chucking, particles may lead to chucking non-flatness. For lithographic systems using exposure wavelengths beyond the current 193 nm generation, such as 157 nm, 13.4 nm Extreme Ultraviolet, and ˜1 nm X-ray there is no known pellicle solution that is simultaneously transparent to the exposure energy and protective. Similarly, there is no pellicle solution for lithographic exposure systems using particle beams such as the electrons used in Electron Projection Lithography (EPL) and Low Energy Electron Projection Lithography (LEEPL) or the ions used in Ion Project Lithography (IPL).
An emerging problem in particular is contaminants that are adsorbed on the surface of the mask, such as sulfates and ammonia. At the high-energy exposure conditions used in the advanced steppers, these contaminants can form particles that grow large enough to print on the wafer. The contaminants may be airborne in origin, or introduced to the mask surface during manufacture, transportation and storage. Recent analyses have shown that, even after rinsing by ultra-pure water (UPW) with dissolved hydrogen and ozone, significant surface contamination by organics, fluorinated compounds, sulfur compounds and siloxanes is still able to develop. Such surface contamination is largely from external sources, believed to be airborne contamination introduced after the cleaning.
Current storage pods for masks have a filter to minimize influences from external airborne particles, but not from the chemical contamination. A method for eliminating particles as well as adsorbed contaminants on the surface of the mask without the protective pellicle is needed.